This invention relates to a protective circuit, and more specifically to a protective circuit for a direct-coupled amplifier and also for a load device coupled to the amplifier.
It is a usual practice to provide a direct-coupled amplifier with a protective circuit for protecting a load device, such as a loudspeaker, connected to the amplifier from being damaged due to d-c (direct current) voltage deviation at the intermediate point of the amplifier, and also for protecting output transistors of the amplifier from being damaged due to excessive currents caused by short-circuiting of output terminals of the amplifier or its load device.
A conventional protective circuit will now be described with reference to FIG. 1 of the accompanying drawings. A direct-coupled amplifier 1 comprises output transistors 2 and 3, and resistors 10 and 14, which constitute a single-ended push-pull output stage. The collectors of the transistors 2 and 3 are respectively connected to a positive and negative d-c power sources (+B, -B), and the emitters thereof are connected through the series-connected resistors 10 and 14. The amplifier 1 is connected, through its intermediate point or junction A and its output terminal OUT, to a load device (not shown) such as a loudspeaker for driving same. The d-c level at the point A is substantially zero volt when the amplifier 1 is functioning normally.
The prior art of FIG. 1 is provided with two protective circuits. One is for protecting the load device from being damaged due to the d-c voltage deviation at the intermediate point A, and comprises a low pass filter 6 consisting of a resistor and a capacitor 5, an absolute value amplifier 7, a driver 8, and a relay 9 driven by the driver 8. If the d-c voltage at the point A deviates due to trouble or malfunction in the amplifier 1, then the low pass filter 6 receives the deviated d-c voltage through the resistor 4 directly coupled to the point A. The filtered output from the low pass filter 6 is fed to the driver 8 via the absolute value amplifier 7, and actuates the relay 9 for isolating the point A from the output terminal OUT by opening relay contacts 9-1. Thus, the load device can be protected from being damaged by the d-c deviation at the point A.
The other protective circuit of the prior art is denoted by a reference numeral 13, and comprises two transistors 11 and 12 and three resistors (no numerals). This protective circuit 13 detects the current from the transistor 2 by detecting the voltage across the resistor 10. If the current flowing through the transistor 2 abnormally increases due to short-circuiting of the load device, the protective circuit 13 detects the excessive current and actuates the relay 9 via the driver 8, thereby isolating the output transistors 2 and 3 from the output terminal OUT.
Thus it will be understood that as the prior art is provided with the two different protective circuits, one for protecting the load device and the other for protecting the output transistors. The prior art is complex and expensive even though the two protective circuits share the same driver 8 and relay 9.